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<p>每天一篇面试小知识</p>
</blockquote>
<p><strong>本篇着重介绍一下 Spring 循环依赖以及三级缓存</strong></p>
<hr>
<p><img src="https://gcore.jsdelivr.net/gh/PaulGeorge123/cloudimg@master/img/all/Snipaste_2021-07-21_08-45-33.png"></p>
<p><strong>写在前面</strong></p>
<p><font color=#008000>什么是循环依赖？</font></p>
<p>简单的说就是 A 依赖 B，B 依赖 A 这样就构成了循环依赖。</p>
<p><font color=#008000>具体说一说 spring 是如何解决循环依赖的吧</font></p>
<p>让我好好想一想……</p>
<span id="more"></span>



<h3 id="什么是循环依赖"><a href="#什么是循环依赖" class="headerlink" title="什么是循环依赖"></a>什么是循环依赖</h3><p>所谓的循环依赖是指，A 依赖 B，B 又依赖 A，它们之间形成了循环依赖。或者是 A 依赖 B，B 依赖 C，C 又依赖 A，形成了循环依赖。更或者是自己依赖自己。</p>
<p>举个图解荔枝：</p>
<img src="https://gcore.jsdelivr.net/gh/PaulGeorge123/cloudimg@master/img/all/Snipaste_2021-07-21_22-16-10.png" style="zoom:90%;" />



<p>举个代码荔枝：</p>
<figure class="highlight java"><table><tr><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="keyword">class</span> <span class="title class_">BeanB</span> &#123;</span><br><span class="line">     <span class="keyword">private</span> BeanA beanA;</span><br><span class="line"></span><br><span class="line">     <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">setBeanA</span><span class="params">(BeanA beanA)</span> &#123;</span><br><span class="line">         <span class="built_in">this</span>.beanA = beanA;</span><br><span class="line">     &#125;</span><br><span class="line"> &#125;</span><br><span class="line"></span><br><span class="line"><span class="keyword">public</span> <span class="keyword">class</span> <span class="title class_">BeanA</span> &#123;</span><br><span class="line">    <span class="keyword">private</span> BeanB beanB;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">setBeanB</span><span class="params">(BeanB beanB)</span> &#123;</span><br><span class="line">        <span class="built_in">this</span>.beanB = beanB;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>配置文件：</p>
<figure class="highlight xml"><table><tr><td class="code"><pre><span class="line"><span class="tag">&lt;<span class="name">bean</span> <span class="attr">id</span>=<span class="string">&quot;beanA&quot;</span> <span class="attr">class</span>=<span class="string">&quot;com.ase.BeanA&quot;</span>&gt;</span></span><br><span class="line">  <span class="tag">&lt;<span class="name">property</span> <span class="attr">name</span>=<span class="string">&quot;beanB&quot;</span> <span class="attr">ref</span>=<span class="string">&quot;beanB&quot;</span>/&gt;</span></span><br><span class="line"><span class="tag">&lt;/<span class="name">bean</span>&gt;</span></span><br><span class="line"></span><br><span class="line"><span class="tag">&lt;<span class="name">bean</span> <span class="attr">id</span>=<span class="string">&quot;beanB&quot;</span> <span class="attr">class</span>=<span class="string">&quot;com.ase.BeanB&quot;</span>&gt;</span></span><br><span class="line">  <span class="tag">&lt;<span class="name">property</span> <span class="attr">name</span>=<span class="string">&quot;beanA&quot;</span> <span class="attr">ref</span>=<span class="string">&quot;beanA&quot;</span>/&gt;</span></span><br><span class="line"><span class="tag">&lt;/<span class="name">bean</span>&gt;</span></span><br></pre></td></tr></table></figure>

<p>按照上面的例子，Spring 启动后，读取如上的配置文件，会按顺序先实例化 A，但是创建的时候又发现它依赖了 B，接着就去实例化 B ，同样又发现它依赖了 A ，从而导致了无限循环，淦！</p>
<p><strong>尝试思考</strong></p>
<p>Spring 实例化对象分两步：</p>
<p>第一步，会先创建一个原始对象，只是没有设置属性，可以理解为”半成品”—— 官方叫 A 对象的早期引用（EarlyBeanReference）；</p>
<p>第二步，当实例化 B 的时候发现依赖了 A， B 就会把这个“半成品”设置进去先完成实例化，既然 B 完成了实例化，所以 A 就可以获得 B 的引用，也完成实例化了，这其实就是 Spring 解决循环依赖的思想；</p>
<hr>
<h3 id="循环依赖发生的时机"><a href="#循环依赖发生的时机" class="headerlink" title="循环依赖发生的时机"></a>循环依赖发生的时机</h3><h4 id="Bean-实例化"><a href="#Bean-实例化" class="headerlink" title="Bean 实例化"></a>Bean 实例化</h4><p>主要分为三步，如图：</p>
<img src="https://gcore.jsdelivr.net/gh/PaulGeorge123/cloudimg@master/img/all/Snipaste_2021-07-21_22-33-22.png" style="zoom:80%;" />



<p>问题出现在：第一步和第二步的过程中，也就是填充属性 / 方法的过程中</p>
<h4 id="Spring-如何解决的"><a href="#Spring-如何解决的" class="headerlink" title="Spring 如何解决的"></a>Spring 如何解决的</h4><ul>
<li>Spring 为了解决单例的循环依赖问题，使用了 <strong>三级缓存</strong> ，递归调用时发现 Bean 还在创建中即为循环依赖</li>
<li>单例模式的 Bean 保存在如下的数据结构中：</li>
</ul>
<figure class="highlight java"><table><tr><td class="code"><pre><span class="line"><span class="comment">/**一级缓存，用于存放完全初始化好的 bean，从该缓存中取出的 bean 可以直接使用*/</span></span><br><span class="line"><span class="keyword">private</span> <span class="keyword">final</span> Map&lt;String, Object&gt; singletonObjects = <span class="keyword">new</span> <span class="title class_">ConcurrentHashMap</span>&lt;&gt;(<span class="number">256</span>);</span><br><span class="line"></span><br><span class="line"><span class="comment">/** 二级缓存：存放原始的 bean 对象（尚未填充属性），用于解决循环依赖 */</span></span><br><span class="line"><span class="keyword">private</span> <span class="keyword">final</span> Map&lt;String, Object&gt; earlySingletonObjects = <span class="keyword">new</span> <span class="title class_">HashMap</span>&lt;&gt;(<span class="number">16</span>);</span><br><span class="line"></span><br><span class="line"><span class="comment">/** 三级级缓存：存放 bean 工厂对象，用于解决循环依赖 */</span></span><br><span class="line"><span class="keyword">private</span> <span class="keyword">final</span> Map&lt;String, ObjectFactory&lt;?&gt;&gt; singletonFactories = <span class="keyword">new</span> <span class="title class_">HashMap</span>&lt;&gt;(<span class="number">16</span>);</span><br><span class="line"></span><br><span class="line"><span class="comment">/**</span></span><br><span class="line"><span class="comment">bean 的获取过程：先从一级获取，失败再从二级、三级里面获取</span></span><br><span class="line"><span class="comment"></span></span><br><span class="line"><span class="comment">创建中状态：是指对象已经 new 出来了但是所有的属性均为 null 等待被 init</span></span><br><span class="line"><span class="comment">*/</span></span><br></pre></td></tr></table></figure>

<ul>
<li>一级缓存：singletonObjects，存放完全实例化属性赋值完成的 Bean，直接可以使用。</li>
<li>二级缓存：earlySingletonObjects，存放早期 Bean 的引用，尚未属性装配的 Bean</li>
<li>三级缓存：singletonFactories，三级缓存，存放实例化完成的 Bean工厂。</li>
</ul>
<h4 id="循环依赖的解决流程图"><a href="#循环依赖的解决流程图" class="headerlink" title="循环依赖的解决流程图"></a>循环依赖的解决流程图</h4><p><img src="https://gcore.jsdelivr.net/gh/PaulGeorge123/cloudimg@master/img/all/Snipaste_2021-07-22_00-13-55.png"></p>
<hr>
<h4 id="A、B-相互依赖"><a href="#A、B-相互依赖" class="headerlink" title="A、B 相互依赖"></a>A、B 相互依赖</h4><p>检测循环依赖的过程如下：</p>
<ul>
<li><p>A 创建过程中需要 B，于是 <strong>A 将自己放到三级缓里面</strong> ，去实例化 B</p>
</li>
<li><p>B 实例化的时候发现需要 A，于是 B 先查一级缓存，没有，再查二级缓存，还是没有，再查三级缓存，找到了</p>
<ul>
<li><strong>然后把三级缓存里面的这个 A 放到二级缓存里面，并删除三级缓存里面的 A</strong></li>
<li>B 顺利初始化完毕，<strong>将自己放到一级缓存里面</strong>（此时B里面的A依然是创建中状态）</li>
</ul>
</li>
<li><p>然后回来接着创建 A，此时 B 已经创建结束，直接从一级缓存里面拿到 B ，然后完成创建，<strong>并将自己放到一级缓存里面</strong></p>
</li>
<li><p>如此一来便解决了循环依赖的问题</p>
</li>
</ul>
<p><strong>总结</strong></p>
<p>先让最底层对象完成初始化，通过三级缓存与二级缓存提前曝光创建中的 Bean，让其他 Bean 率先完成初始化。</p>
<h3 id="注意"><a href="#注意" class="headerlink" title="注意"></a>注意</h3><p><font color=#FF8C00>Spring 还是有一些无法解决的循环依赖！</font></p>
<p>使用构造器注入其他 Bean 的实例，这个就没办法了，要手动改代码。</p>
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